Re: regex.m4: how do I know that the included regex is used?

[Jim Meyering]

Bruno Haible wrote:

> Sam Steingold wrote:
>> If I prepend -I$(srcdir)/gllib to CPPFLAFS, then
>>
>> #include <regex.h>
>>
>> will read the gnulib header, not the system header.
>
> Yes.
>
>> How can you be sure that this will not lead to any problems given that
>> the libgnu.a does _not_ include regex.o?
>
> Because regex.m4 guarantees that. It tests for re_compile_pattern, and
> only glibc systems have this function (including BeOS, which counts as
> a glibc from ca. 1998). We are relying on the fact that glibc will maintain
> backward compatibility for regex, like it did even when the regex code was
> completely rewritten by Isamu Hasegawa.
>
>> If course, you can claim that, if gnulib regex.h were incompatible with
>> the system one, then configure would have detected that and included
>> gnulib regex in libgnu.a.
>
> Exactly.
>
>> In that case, why the file fnmatch.in.h not called fnmatch.h?
>
> If gnulib would only offer the 'fnmatch-gnu' module, this would make sense.
> But it also offers the 'fnmatch' module, which does not insist on a GNU
> like implementation of fnmatch().
>
>> At any rate, if gnulib regex is included in libgnu.a, then regex.h
>> should be copied (or linked) to $(builddir) or $(builddir)/gllib.
>> The current situation is clearly wrong: regex is the only gnulib package
>> which requires prepending -I$(srcdir)/gllib to CPPFLAFS!
>
> It is the package's responsibility to be consistent about $(builddir)
> vs. $(srcdir). Don't use
>    -I$(srcdir)/gllib
> or
>    -I$(builddir)/gllib
> in isolation. Always use
>    -I$(builddir)/gllib -I$(srcdir)/gllib
> so that -I options that come before it take precedence, -I options that come
> after have lower precedence, and files in $(builddir)/gllib hide possible
> (accidentally leftover) files in $(srcdir)/gllib.
>
> This is something that IMO ought to be mentioned in the Autoconf manual.
> I have seen many GNU packages do this wrong. What do you think (Paul,
> Eric, Jim, Ralf)?

Good point.
I know I've gotten it wrong a few times.